Among image capturing apparatuses having solid-state image sensor, such as CCD or CMOS sensor, some of these image capturing apparatuses have so-called moving image recording function capable of sequentially recording image signals consecutively read from the image sensors.
Further, as a general method of auto focus detection and adjustment in image capturing apparatuses using a light beam that passes through a photographing lens, a contrast detection method and a phase difference detection method are known.
The contrast detection method is a method which is often adopted to a video movie device for capturing a moving image and an electronic still camera, and an image sensor for capturing an image is also used as a sensor for focus detection. In the contrast detection method, output signals from an image sensor, especially information on a high frequency component (contrast information) is given attention, and the evaluation value of the high frequency component is maximized to attain an in-focus position of the image pickup lens. However, in the contrast detection method, the evaluation value is obtained by slightly moving the image pickup lens so as to maximize the high-frequency components, it is not suitable for high-speed focus control operation.
The phase difference detection method, on the other hand, is often used in single lens reflex cameras, and is the technique that has contributed most to the practical use of autofocus (AF) single lens reflex cameras. According to the phase difference detection method, a light beam that has passed through the exit pupil of the photographing lens is divided into two, and the two divided light beams are respectively received by a pair of focus detection sensors. The amount of defocus for the photographing lens in the focusing direction is directly determined by detecting the difference between the output signals according to the amount of light received, or in other words, an amount of relative positional shift in the direction in which the light beam is divided. Accordingly, once a charge accumulation operation is executed by the focus detection sensors, the amount of defocus and the direction of defocus can be obtained, making it possible to perform a high-speed focus adjustment operation. However, in order to divide light beam that has passed through the exit pupil of the photographing lens into two and obtain signals corresponding to each of two light beams, it is common to provide a light splitting mechanism, such as a quick return mirror and a half mirror, in the image sensing optical path, and provide a focus detection optical system and an AF sensor in the downstream of the light path splitting device. Accordingly, there are disadvantages in the phase difference detection method such that the apparatus becomes large and expensive. In addition, the phase difference detection method cannot be used when displaying a live view image, since a quick return mirror is withdrawn from the light path.
In order to overcome the above disadvantages, a technique has also been disclosed in, for example, Japanese Patent Laid-Open No. 2000-156823, wherein a pupil division mechanism is provided to some of the light-receiving elements (pixels) of an image sensor by offsetting the sensitivity region of the light-receiving portion with respect to the optical axis of the on-chip microlens. These pixels are used as focus detection pixels, and arranged with a prescribed spacing between image pickup pixel groups to perform phase difference focus detection, thereby realizing a high-speed AF using a phase difference method even during using an electronic view finder (EVF) as well as during capturing a moving image.
Further, the following method is disclosed in order to prevent degradation of an image due to moiré and so on when using an EVF and when capturing a moving image. Namely, in an added reading mode for reading pixel signals from an image sensor while adding them, if there are focus detection pixel signals, the reading of pixels signals is performed so that the pixel signals of image pickup pixels and pixel signals of focus detection pixels do not mix (e.g., Japanese Patent Laid-Open No. 2010-20055).
However, in the Japanese Patent Laid-Open Nos. 2000-156823 and 2010-20055, the portions where the focus detection pixels are arranged correspond to portions where image pickup pixels do not exist, so image information is generated through interpolation using information from the peripheral image pickup pixels. Accordingly, when a ratio of the number of the focus detection pixels to the number of the image pickup pixels is sufficiently small, degradation of an image is small, but degradation of an image increases as a ratio of the number of the focus detection pixels to the number of the image pickup pixels increases.
In the meantime, in a case where an EVF mode or a moving image capture mode is set, it is known to read pixel signals from part of the pixels of the image sensor to achieve high speed reading of an image. However, an increase in a ratio of the number of the read focus detection pixels to the number of the read image pickup pixels when reading part of the pixels in comparison to when reading all of the pixels can greatly affect the image quality.
Further, in a case where an area where focus detection pixels are arranged is limited to the central part and its vicinity of the image pickup area where the optical condition is good and relatively high focus detection precision can be achieved and a uniform color plane object is sensed, the following problem is posed. Namely, degradation of an image is tend to be conspicuous at a boundary between an area where focus detection pixels are arranged and an area where no focus detection pixel is arranged due to an correction error using the image pickup pixels arranged around the focus detection pixels.